The present invention relates to a stencil printing machine wherein a perforated stencil sheet is mounted onto a printing drum against which a print sheet is pressed during transfer thereof to allow ink to be transferred to the print sheet through a perforated area of the stencil sheet to generate a desired image thereon, and more particularly to a stencil printing machine effectively preventing paper dusts from being adhered to the stencil sheet mounted onto the printing drum.
Several researches and developments have been undertaken to provide a stencil printing machine of the type which enables printing with the use of a printing drum and a rotary back press roller, a typical example of which is disclosed in FIG. 9 which shows an essential part of a structure of the prior art stencil printing machine.
In FIG. 9, the stencil printing machine 10 includes a paper feed section 12 adapted to feed a print sheet S one by one from a stack of print sheets (not shown), and a print section 14 which enables printing on the print sheet S, which has been fed from the paper feed section 12, to reproduce a desired image thereon.
The paper feed section 12 includes plural paper feed roller pairs 12a, 12b which are arranged to be held in press contact with the print sheet S, creating frictional pressure during rotation of the roller pairs 12a, 12b to cause the print sheet S to be transferred to the print section 14 at predefined timings. The print section 14 includes a printing drum 16, and a back press roller 18 located in close proximity thereto and serving as a rotary printing press member for imparting printing pressure to the printing drum 16. In particular, the printing drum 16 and the back press roller 18 are rotatably supported under a condition wherein respective outer circumferential peripheries are partially located in substantially close proximity to one another. The printing drum 16 carries on its outer circumferential periphery a stencil clamping base 20 which clamps a leading edge of a stencil sheet 22. The stencil sheet 22 has a perforated image area or pattern which is formed by perforating with a thermal printing head based on image data. The outer circumferential periphery of the printing drum 16 also carries a flexible screen 24 over an area except for the stencil clamping base 20, with the flexible screen 24 being constituted with an ink permeable porous sheet. An inner press roller 26 is located inside the printing drum 16, which is moveable between a press contact position wherein the inner press roller 26 remains in press contact with an inner peripheral wall of the screen 24 and a wait position wherein the inner press roller 26 is held out of press contact with the inner peripheral wall of the screen 24. An outer periphery of the inner press roller 26 is supplied with ink, with the thickness of an ink layer adhered onto the inner press roller 26 being regulated with a gap formed by a doctor roller 28 to maintain the quantity of the ink to be applied to the screen 24 at a given constant level substantially at all times in the operation.
In the operation with such a structure, the perforated stencil sheet is made by perforating the stencil sheet 22 over a given area thereof on the basis of image data to produce a desired perforated area, and the perforated stencil sheet 22 is mounted onto the outer circumferential periphery of the printing drum 16. The printing drum 16, to which the stencil sheet 22 is mounted, is rotated in synchronism with the back press roller 18 as shown by an arrow in FIG. 9, with the print sheet S being transferred between the printing drum 16 and the back press roller 18 from the paper feed section 12. In this instance, the inner press roller 26 is moved to the press contact position to cause the screen 24, which adjacently has the stencil sheet 22, to be expanded outward during transfer of the print sheet S for thereby allowing the print sheet S to be held in press contact between the screen 24 and the back press roller 18 while transferring the print sheet S. During such a transfer step of the print sheet S in press contact with the screen 24 and the back press roller 18, the ink is transferred from the inside of the screen 24 to the print sheet S through the perforated area of the stencil sheet 22, reproducing the desired image on the print sheet S in dependence on the perforated area of the stencil sheet 22.
Due to further investigations performed by the present inventors, it is thought that the print sheet S encounters paper dusts that are produced due to friction caused with the paper feed roller pairs during travel thereof or paper dusts that are produced when the print sheet S has been cut and are adhered to the print sheet S. With such paper dusts adhered onto the print surface of the print sheet S, the paper dusts are transferred onto the stencil sheet 22 mounted onto the printing drum 16 when the print sheet S is pressed against the printing drum 16 such that the paper dusts are adhered to the stencil sheet 22. The paper dusts, that are separated from the print sheet S in a floating state, are adhered to the surface of the stencil sheet 22.
The paper dusts adhered to the stencil sheet 22 readily absorb the ink, thereby causing inadequate quantity of the ink to be transferred onto the print sheet S. As a result, the printed sheet S tends to have a thin area (i.e., a white area) of image corresponding to that of the stencil sheet adhered with the paper dusts. Also, in the event that the print sheet S and the paper dusts remaining thereon are sufficiently wet, a large quantity of the ink permeates into the paper dusts, and such ink is transferred onto the print sheet S. This causes the print sheet S to be transferred with more quantity of the ink than required, with the printed image being reproduced at more density in an area (i.e., a black area) corresponding to that of the stencil sheet adhered with the paper dusts than required.
That is, it is thought that, when the paper dusts are adhered to the stencil sheet 22 of the printing drum 16, the printed image are apt to involve the white and black areas, with a deteriorated quality in the printed image reproduced on the print sheet S. Such a deterioration in the print image becomes more conspicuous especially when the print section employs oily ink such as UV ink.
The present invention has been made through the above investigations by the present inventors and has an object to provide a stencil printing machine which prevents paper dusts from being adhered to a stencil sheet mounted onto a printing drum for thereby enabling a print sheet to be reproduced with a print image at a high quality.
According to one aspect of the present invention, there is provided a stencil printing machine which comprises: a paper feed section feeding a print sheet in a paper feed path, the print sheet being allowed to have paper dusts thereon; a print section to which the print sheet is transferred, and including a printing drum carrying thereon a stencil sheet having a perforated image area formed on the basis of image data and a rotary printing press member located in close proximity to an outer circumferential periphery of the printing drum to impart printing pressure thereto to allow the print sheet to be transferred in press contact between the printing drum and the rotary printing press member such that printing ink is transferred onto the print sheet to generate a desired image thereon through the perforated image area; and a liquid applying mechanism applying liquid, which is permeable to the paper dusts, onto a print surface of the print sheet to allow the paper dusts to be permeated with the liquid at an upper stream position than a position where the printing drum and the rotary printing press member are in press contact with each other in the paper feed path.
According to another aspect of the present invention, there is provided a stencil printing machine comprises: a paper feed section feeding a print sheet in a paper feed path, the print sheet being allowed to have paper dusts thereon; a print section to which the print sheet is transferred, and including a printing drum carrying thereon a stencil sheet having a perforated image area formed on the basis of image data and a rotary printing press member located in close proximity to an outer circumferential periphery of the printing drum to impart printing pressure thereto to allow the print sheet to be transferred in press contact between the printing drum and the rotary printing press member such that printing ink is transferred onto the print sheet to generate a desired image thereon through the perforated image area; and a liquid applying mechanism applying liquid, which is permeable to the paper dusts, onto the stencil sheet from which the liquid is transferred to a print surface of the print sheet to allow the paper dusts to be permeated with the liquid.
According to another aspect of the present invention, there is provided a stencil printing machine comprises: a paper feed section feeding a print sheet in a paper feed path, the print sheet being allowed to have paper dusts thereon; a print section to which the print sheet is transferred, and including a printing drum carrying thereon a stencil sheet having a perforated image area formed on the basis of image data and a rotary printing press member located in close proximity to an outer circumferential periphery of the printing drum to impart printing pressure thereto to allow the print sheet to be transferred in press contact between the printing drum and the rotary printing press member such that printing ink is transferred onto the print sheet to generate a desired image thereon through the perforated image area; and a liquid applying mechanism applying liquid, which is permeable to the paper dusts, onto a print surface of the print sheet to allow the paper dusts to be permeated with the liquid at a position where the printing drum and the rotary printing press member are in press contact with each other and/or at an upper stream position than the position where the printing drum and the rotary printing press member are in press contact with each other in the paper feed path.